1. Field of the Invention
The present invention relates to a head up display device, and more particularly, to an external mounting type head up display device for a vehicle, which can be mounted to a vehicle irrespective of kind of vehicle.
2. Discussion of the Related Art
There are several safety systems under development for the purpose of effectively providing a driver with information about vehicle driving states and surrounding circumstances to secure safe driving. Of the safety systems, a head up display (HUD) device is highlighted.
A HUD device is used to provide a driver with information about vehicle driving states and navigation to the front of a driver, i.e., within a driver's view range while driving a vehicle.
An initially developed HUD device was installed in an airplane, especially a fighting plane, for the purpose of providing a pilot with flight information while flying. A HUD device for a vehicle has been developed by applying an initial HUD device for an airplane to a vehicle.
For example, in a case where a driver drives a vehicle at a speed of approximately 100 km/h, on the assumption that it takes approximately two seconds for the driver to divert observation to the road after checking information on a dashboard, the vehicle travels about 55 m in the mean time, which may cause a car accident.
As one of methods for decreasing accident risk, a HUD device for a vehicle has been developed. A HUD device for a vehicle displays dashboard information (speed, driving distance, RPM, etc.) on a windshield within a driver's view range, thereby enabling the driver to perceive important information about vehicle driving states without taking eyes off the road. Accordingly, the driver can drive a vehicle safely.
FIG. 1 is a view illustrating an example of a constitution of a conventional head up display device.
As shown in the drawing, a conventional head up display device includes a display unit 11 to display information about vehicle driving states and navigation, a projection unit 13 to project an image outputted from the display unit 11, and a combiner 15 mounted on a windshield 14 to reflect the image projected from the projection unit 13 to a driver in a holographic image form. Depending on the necessities, a mirror system 12, which changes an optical path of the image, may be installed between the display unit 11 and the projection unit 13.
The display unit 11 may include a light source using an ultra high pressure (UHP) lamp, an LED or a laser, and a display panel capable of microdisplay and scan display, such as an LCD, an LCOS or a DMD. In this case, the image outputted from the display unit 11 should have high surface brightness so that the image is visible in broad clear daylight. The projection unit 13 is an optical system that projects the image from the display unit 11 to the combiner 15. The projection unit 13 generally includes plural lenses. The combiner 15 is provided with a recording layer which is formed with a hologram pattern so that the combiner 15 has a high transmittance for light incident from the exterior of the vehicle and has a high reflectance for light incident from the display unit 11 and the projection unit 13.
According to the conventional head up display device as structured above, as shown in FIG. 2, the information about vehicle driving states and navigation is outputted from the display unit 11 in the image form, and the outputted image is projected to the windshield 14 by the projection unit 22 (illustrated to include a mirror system). The image projected to the windshield 14 is reflected from the combiner 15 to the driver's eyes. The image transmitted to the driver's eyes appears as a virtual image (HUD image) 21 at a predetermined distance, preferably 2.4 m or more, from the driver, with a lookdown angle θ from the horizontal sight line. Accordingly, the driver sees the virtual image overlapped with outside scenery viewed through the windshield. It is preferred that the lookdown angle θ is set within a range from approximately 10 to 15°, in order for the driver to easily observe the image of the information about the vehicle driving states and the like without confusing the image with the outside scenery.
However, as shown in FIG. 2, the conventional head up display device as structured above is configured such that the at least display unit 11 and projection unit 22 are mounted inside the dashboard of the vehicle. In other words, the conventional head up display device is manufactured adequately only for an internal mounting type which should be mounted inside the vehicle in advance before the vehicle is released in the market. But, there are few conventional head up display devices which can be purchased separately and mounted to the vehicle after the vehicle is released in the market. Also because a portable and external mounting type head up display device is mostly mounted in a space between a windshield and a dashboard, it is required to develop a head up display device which can be mounted also in a vehicle in which the space between the windshield and the dashboard is small.
Moreover, as a volume of the head up display device increases, the reflected image shakes more by vehicle vibration, and thus an image quality is degraded. Also because the image is projected using the entire mirror, there is a limitation in manufacturing the conventional head up display device compactly. Still further, in a case where a large element is used, the conventional head up display device needs a cooling system for solving a temperature rise problem, which results in a decrease in reliability and an increase in cost.